The number of electrons lost or gained or shared by atoms during chemical bond formation decides the valency of an element. Similarly the oxidation state of an element is obtained by the number of electrons it can gain or loose to form an octet configuration.
The electronic configuration of an element helps to find out the oxidation state of that element to form a particular ion of that element.
When s block elements react chemically they donate electron and show positive oxidation states . When alkali metals undergo chemical reactions, they show +1 oxidation state and alkaline earth metals show +2 oxidation state. These elements show definite oxidation states only.
P block elements show positive oxidation state and negative oxidation state.
Oxidation state remains constant in each group. For e.g. group 13 = +3, group 14= +4, group 15 = -3, etc.